Articles comprising security images are well known. Security images generally comprise an image which is invisible or otherwise undetectable under ambient conditions, and which can be rendered visible or detectable by application of a suitable stimulus; or alternatively, the image may change from one colour under ambient conditions to another colour upon application of a stimulus. The stimulus may, for example, be electromagnetic radiation or heat.
Security images may be produced by coating security inks or compounds onto a substrate. Examples of known compounds which when coated onto a substrate provide a security image include photochromic compounds which generally change from colourless to coloured upon the application of ultraviolet light, and thermochromic compounds which generally change from colourless to coloured upon the application of heat.
Articles which include security images are useful in many areas of industry, for example in packaging, identification cards, and labels. Such articles may comprise a further printed image, in addition to the security image. It is useful to provide packaging which includes a security image invisible to a user under ambient conditions, but which can be rendered visible upon application of a stimulus; for example, if a customs and excise official wishes to check whether imported goods are genuine or counterfeit. If the packaging includes the security image, rendered visible or otherwise detectable by a suitable stimulus, the customs and excise official can determine that the packaging, and hence the goods, are not counterfeit. Likewise, it is advantageous to provide an identification card in which a security image is invisible or a defined colour under ambient conditions, but which can be rendered visible or detectable, or change colour upon application of a stimulus in order to prove the identity of a user of the identity card, in order to determine that the identity card is genuine. There are many known examples of such security images, for example, in the applicant's co-pending applications PCT/GB2005/001763 and PCT/GB2005/001766.
In the manufacture of bank notes, it is desirable to include as many security features as possible, which may include multiple security images using a variety of compounds capable of changing colour upon application of a stimulus or stimuli (including movement of the bank note to change viewing angle), or turning coloured from colourless, or vice versa.
In many countries, officials and state authorities use apparatus, such as third party verifiers, which detect the percentage radiation absorbance and/or reflectance at a wavelength of approximately 800-900 nm (in the infrared region), to detect whether specific security images comprising compounds which absorb infrared radiation between 800-900 nm are present; and hence help to determine whether or not a bank note is genuine or counterfeit.
It is desirable to provide bank notes which contain security images comprising compounds capable of exhibiting 50% or less radiation reflectance at approximately 800-900 nm. Many bank notes include carbon black as a pigment which possesses the characteristic of less than 50% light reflectance at 800-900 nm. Unfortunately, in order to provide a suitably strong image, with the required radiation reflectance characteristics at 800-900 nm, carbon black is generally needed in a concentration which produces a dull grey image in the positions where the carbon black are located, when coated at concentrations generally used (for example, 3% w/w of the total weight of the ink dispersion laid down on the substrate paper for bank notes). Bank note counterfeiters recognise from the dull grey image that carbon black is present in bank notes, and commonly now use carbon black in order to prevent their counterfeit bank notes being detected as counterfeit when third party verifiers are used to verify the radiation reflectance at 800-900 nm.
It would therefore be advantageous to provide a security image on a bank note or any other article requiring a security image, in which the security image includes one or more compounds having, a 50% or less radiation reflectance at 800-900 nm at a given concentration within an image (for example around 850 nm), and in which a strongly coloured image is not created by the compound(s) utilised. It would be particularly advantageous to provide such a compound for inclusion in a security image in which the compound produces a substantially colourless security image, but which has 50% or less radiation reflectance at 800-900 nm. Most preferably 40% radiation reflectance in the 800-900 nm region is desired.
Further, it would be advantageous to provide a security image on a banknote or other imaged article requiring a security image which could be detected at wavelengths other than 800 to 900 nm, for example over the range 700 to 1500 nm. This would enable an additional security feature to be introduced which is currently not known to counterfeiters. Radiation absorbed or reflected at wavelengths of 750 to 1500 nm falls within the infrared radiation region of the electromagnetic spectrum.
Infrared-absorbing compounds currently used in security imaging include various complex organic compounds, in addition to carbon black. Some of these organic compounds do not create a strongly coloured image; for example see the applicant's co-pending application GB 0602821.1. However such compounds have other disadvantages.
Many known organic infrared-absorbing compounds are complex molecules: their synthesis is often difficult and can be very expensive. Due to the time taken to prepare the compounds, supply problems may result. In addition, such compounds have been found to exhibit poor light fastness. Over time, exposure to daylight leads to decomposition and the usefulness of such compounds as infrared absorbers diminishes.